Method and apparatus for dip-coating articles



NOV 5, 1957 1 E. M. RANSBURG 2,812,269

METHOD AND APPARATUS FOR DIP-COATING ARTICLES Filed March 24,` 1952 .IWION w21-000 N N v n ON @W @1% En@ A/forneys 2,812,269 METHOD AND APPARATUS FOR DIP-COATING ARTICLES Edwin M. Ranshurg, Indianapolis, Ind., assignor to Ransburg Electro-Coating Corp., a corporation of Indiana Application March 24, 1952, Serial No. 278,228 12 Claims. (Cl. 117-47) This invention relates to a coating method and apparatus, and more particularly to a dip coating method and apparatus.

A dip coating process and apparatus now known employs a cooling zone above a pool of heated solvent containing liquid coating material; dips the article to be coated in the pool for a period long enough to bring it to a temperature near that of the heated liquid coating material; then raises it out of the pool and holds it in the cooled Zone for a period sufficient to promote a desired amount of rapid vaporization of solvent from the liquid iilm thus provided on the article.

Such a dip coating process, however, not only has the disadvantage of not being adapted to the rapid commercial coating of a plurality of articles in succession, but also even at slow operating speeds frequently results in an unsatisfactory coating (or no coating at all) at seams, beaded portions, or other article crevices and irregularities. In dip coating in the above mentioned manner articles at room temperature entering the zone of solvent vapors in the atmosphere immediately above the surface of the heated pool condense solvents on their surfaces and the solvent which thus gathers in seams and other crevices on certain articles does not mingle with the remainder of the coating material satisfactorily when the article is in the pool, surface tension and other effects in such crevices and small valleys resulting in the solvent remaining pocketed therein. When the article is raised from the pool there are thus portions where the constituents of the liquid coating material (such as the pigments and binders in conventional paints, synthetic enamels and the like) which would otherwise form the eventual solid coating on the surface of the article are present in too small a proportion to the solvent, or not present at all, with resultant unsatisfactory coating of certain areas.

I have devised and am here disclosing and claiming a method and apparatus for coating articles with a heated solvent-containing liquid coating material while employing cooling means for substantially eliminating loss of vapors from above the pool and for promoting volatilization of the solvents from the liquid film on the surface of the article as it leaves the pool of coating material, yet which is particularly adapted for relatively rapid continuous conveyor movement of the articles to be coated, and which overcomes the above mentioned defects in certain portions of the coating and provides a more satisfactory coating over the entire article. In my preferred method and apparatus I employ two cooling zones or sections adjacent'the pool of heated liquid coating material, one through which the article passes as it approaches the pool and another which it traverses after it has been dip coated; I make the latter in such form that the path of travel of the article is at a substantial angle to the vertical, as 60, and cooling means is provided above the article during its travel as well as elsewhere around it to get the optimum volatilization of solvent therefrom, this latter cooling zone preferably being of an extent considerably greater than the first zone entered by the article; I avoid any undesired drippage on the article from the overhead cooling means by providing drip receiving means as a trough, preferably itself heated to avoid condensation on the lower surface thereof; and I pre-heat each article id Succession .as it approaches the pool, and be* Zizg Patented Nov. 5, 1957 "itc v fore it enters the first cooling section, to a temperature in the neighborhood of the liquid coating material in the pool, so that the article enters the pool with a temperature at or very'near that of the liquid material therein and does not condense solvent on itself prior to dipping therein. The preferred form of the apparatus also contemplating a heating section arrangement for providing the bake or final hardening of the coating on the articles after they have left the second cooling section following their dip in the pool of material.

Other features and advantages of my invention will be apparent from the following specification and the drawing, which is a vertical sectional view of apparatus embodying my invention, partly schematic in nature.

While one form of apparatus is illustrated in the accompanying drawing and will be described hereafter as providing means for practising the present invention, it will be understood that this is a representative embodiment only. It will also be understood that forms of dip coating apparatus other than the particular one illustrated may-be utilized for accomplishing the purposes and practising the methods of the present invention, and it is to be understood that other embodiments may be utilized Without departing from the contemplated scope of the present invention and that no limitations are to be implied from the specific description hereinafter provided.

In the particular embodiment of my invention as illustrated herewith, articles 10 are suspended on suitable carriers 11 on a conveyor schematically indicated as 12. During operation articles thus carried by the conveyor are moved in succession, preferably continuously, through a lower or pre-heating portion 13a of a heating section 13 of the apparatus to be heated to a desired temperature before they Vleave the pre-heating Zone through the opening 13a as illustrated. In the apparatus illustrated, the exit opening from the pre-heating zone is immediately above an entrance opening of a first section of cooling means located immediately above a section of the apparatus containing the pool 14 of solvent-containing liquid coating material. The section 15 of the apparatus containing the pool of coating material is here shown as having a jacket comprising spaced walls 15a and 15b between which circulate steam or any other suitable heat transfer fluid which may for example enter the jacket through the conduit 16 and leave the jacket through the conduit 17. Suitable means, as thermostatic controls not illustrated, would be employed to maintain the temperature of the pool of liquid coating material; and during operation of the process, in order to prevent undue dilution by returning solvent condensed in the cooling section, the other constituents of the liquid coating material would he added from time to time to maintain the consistency of the liquid in the pool Within the desired range.

n Adjacent the pool, in the apparatus illustrated, other spaced walls here provide cooling means arranged in two cooling sections. The spaced wall sections 13a and 18b and 19a'and 191; form part of a cooling jacket or cooling means which also includes the spaced walls 20a and 2Gb and 21a and 2lb defining another cooling section or zone. A suitable cooling medium as brine, refrigerant, or the like, may enter through the conduit 22 and be discharged through the conduit 23, with suitable thermostatic controls (not illustrated) maintaining the cooled sections at a desired low temperature, preferably well below room temperature.

It will be noted that as illustrated in the drawing the conveyor 12 causes the articles 10 to -drop vertically from the exit opening of the preheating zone through an opening at the top of the iirst cooling section and then down into the pool of coating material. The heating Y section of the apparatus may be heated by any suitably controlled heating means here illustrated as electric heating elements 24; and the temperature in the preheating zone and the length of time the articles are exposed to such temperature during movement through this section of the apparatus are preferably such that they leave the heating section at a temperature substantially equal to that of the liquid coating material in the pool or even a few degrees above. This enables them to pass through the first cooling section and still, despite any drop in temperature occurring therein, enter the pool of coating material at a temperature equal thereto or only a relatively few degrees lower. This enables the article to pass through the atmosphere of hot vapor immediately above the pool with either no condensation thereon or substantially none, causing a great improvement of uniformity of coating resultingfrom the immediately following dip into the pool. If desired, the articles may be preheated to a temperature substantially above that of` the liquid to more definitely assure absence of condensation on the article. Moreover, preheating enables the conveyor to run relatively rapidly, as the article need not remain in the pool long enough to absorb substantial heat therefrom as has heretofore been the case (generally a half minute to several minutes) but need only remain in the pool just long enough for the liquid material to wet the surface, as only 2 or 3 seconds if desired.

In order to minimize the amount of time which the article would be required to remain in the second cooling ection to promote the desired amount of volatilization of solvent from the coating material film on the surface thereof, I have found it advantageous to arrange the path of travel of the articles in the second section at a substantial angle to the vertical and to provide cooling and condensing means above the article where the rising `warm vapors tend to travel, as well as below and to the sides of the article. Inasmuch as the first cooling section need only provide such cooling action as may be necessary to `keep warm vapors above the pool from passing off into the atmosphere while the second section must achieve not only this purpose but also the desired amount of volatilization of the solvents from the liquid coating material film on the article, the second section is preferably of an extent several times that of theV first cooling section. In order to ensure a substantial length of travel of the article with a cooling surface above it as well as to the sides and beneath it, and also to ensure an arrangement which will avoid excess coating material dripping from one article onto following articles, I arrange the section ata substantial angle to the vertical as 50 or 60 as illustrated.

The presence of this overhead cooling means, particularly in this second section, and the fact that solvent vapors condense thereon, could result in undesirable drippage of solvent on the articles and resultant disturbances in the uniformity of the coating material film. In order to avoid this I provide means for collecting any drippage and discharging it back into the pool in a `manner avoiding any chance of its coming into contact with the articles, this being here illustrated as a transverse trough 2S immediately beneath the juncture line of the walls 18a and a of the cooling means. Since these overhead walls are not only at a substantial angle to the vertical, but also at anappreciable angle to the horizontal, any condensate thereon runs downto this apex or juncture line and drips into the trough where it can be discharged at each end into the pool. In order to prevent any possibility of condensation on the undersurface of the trough and drippage onto the articles, I preferably provide means for heating the trough to a temperature sufiicient to prevent any such condensation, as by use of the electrical heating element 26 illustrated.

While the articles leave at the exit end of the second cooling `zone with practically all or at least the major 'part of the solvent vaporized therefrom, some relatively highspeed coating operations preferably employ a heating or baking operation on the coating, In my preferred apparatus thc conveyor 12 re-enters the heating section of the apparatus and cause the articles, after the dip and initial semi-solidiication of the coating fihn, to receive a further heat or bake. I find it desirable to employ an upper warmer portion of the heating section for this bake operation, this portion 13b of the heating section of the apparatus being maintained at a somewhat higher ternperature than the lower or preheated portions, this being facilitated by its being the upper portion of the heating section.

lvhile l have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

I claim:

l. Apparatus for dip-coating articles, including: a section for holding a pool of heated, solvent-containing liquid coating material adapted to have the articles dipped therein; means adjacent said pool for condensing solvent vapors including an enclosed passageway extending to said pool; a heating section; and a conveyor for moving a plurality of articles in a predetermined path through said heating section, said path dipping into said pool, and passing through said passageway and adjacent said means for condensing solvent vapors, the temperature of said heating section and the time of movement of an article therethrough being such as to heat it sufficiently to prevent any appreciable condensation of solvent vapors, and the movement of the article from the heating section into said pool being sufliciently rapid as to prevent substantial cooling of the article before it is dipped into said pool of coating material.

2. Apparatus for dip-coating articles, including: a section for holding a pool of heated, solvent-containing liquid coating material adapted to have the articles dipped therein; means adjacent said pool for condensing solvent vapors including an enclosed passageway extending to said pool; a heating section, and a conveyor for moving an article through said heating section to raise its temperature to the neighborhood of that of thc heated liquid coating material, dipping `it into said pool, passing through said passageway, and passing it sutliciently near said means to condense solvent vapors, the temperature of said heating section and the time of movement of an article therethrough being such as to heat it suihciently to prevent any appreciable condensation of solvent vapors, and the movement of the article from the heating section into said pool being sufiiciently rapid as to prevent substantial cooling of the article before it is dipped into said pool of coating material.

3. Apparatus for dip-coating articles, including: a section for holding a pool of heated solvent-containing liquid coating material adapted to have the articles dipped therein, said section having an entrance and an exit; a first cooling means at said entrance for condensing solvent vapors; a second cooling means including an enclosed passageway extending to said exit; a heating section; and a conveyor for moving an article through said heating section, through said entrance and vpast said tirst cooling means, dipping it into said pool, and then moving it through said exit and through said passageway past said second cooling means, the temperature of `the heat-` ing section and time of movement of the article therethrough being such as to heat it sufficiently to prevent any appreciable condensation of solvent vapors thereon, and the movement of the article from the heating section into said pool being sufiiciently rapid as to prevent substantial cooling of the article before it is dipped into said pool of coating material.

4. Apparatus for dip-coating articles, including: a section for holding a pool of heated-solvent containing liquid coating material adapted to have the articles dipped therein, saidsection having `an entrance and an exit; a

first cooling `section at said `entrance forrcondensing 'solvent vapors; a second cooling section; a heating section; means forming an enclosed passageway extending between the exit and the second cooling means; and a conveyor for moving an article through said heating section, through said entrance and past said first cooling section, relatively rapidly dipping it into and out of said pool, and then moving it through said exit into said passageway and past said second cooling section, the temperature of the heating section and time of movement of the article therethrough being such as to heat it sufiiciently to prevent any appreciable condensation of solvent vapors thereon, and the movement of the article from the heating section into said pool being suiciently rapid as to prevent substantial cooling of the article before it is dipped into said pool of coating material.

5. Apparatus of the character claimed in claim 5, wherein the passageway is inclined at a considerable angle to the vertical and including cooling means along the upper portion of the passageway to provide cooling means above the article during its movement through said passageway.

6. Apparatus for dip-coating articles, including: a section for holding a pool of heated, solvent-containing liquid coating material adapted to have the articles dipped therein; means adjacent said pool for condensing solvent vapors; means forming an enclosed passageway extending between the pool and said condensing means; a heating section; and a conveyor for moving an article through said heating section, dipping it into said pool, passing through said passageway and sutliciently near said means for condensing solvent vapor to condense solvent vapors from the article, and again moving it through said heating section, the temperature of said heating section and the time of movement of an article therethrough being such as to heat it suiciently to prevent any appreciable condensation of solvent vapors, and the movement of the article from the heating section into said pool being suiciently rapid as to prevent substantial cooling of the article before it is dipped into said pool of coating material.

7. Apparatus for dip-coating articles, including: a section for holding a pool of heated, solvent-containing liquid coating material adapted to have the articles dipped therein; means adjacent said pool for condensing solvent vapors; means forming an enclosed passageway extending between the pool and said condensing means; a heating section means for heating said section to provide therein a higher temperature in the upper portion thereof than in the lower portion thereof; and a conveyor for moving an article through the lower portion of said heating section, dipping it into said pool, passing the article through said passageway and suiiiciently near said means for condensing solvent vapors to condense solvent vapors from the article, and then moving it through the upper portion o f said heating section, the temperature of said heating section and the time of movement of an article therethrough being such as to heat it sutiiciently to prevent any appreciable condensation of solvent vapors, and the movement of the article from the heating section into said pool being sujiciently rapid as to prevent substantial cooling of the article before 't is dipped into said pool of coating material.

8. Apparatus for dip-coating articles, including: a section for holding a pool of heated solvent-containing liquid coating material adapted to have the articles dipped therein, said section having an entrance and an exit; tirst cooling sections at said entrance for condensing solvent vapors; a heating section; means for heating said section to provide therein a higher temperature in the upper portion thereof than in the lower portion thereof; a second cooling section; and a conveyor for moving an article through the lower portion of said heating section and said entrance, dipping it into said pool, moving it through said second cooling section, and then moving it through the upper portion of said heating section, the

temperature of the heating section and time of movement of the article through the lower portion thereof being such as to heat it sufficiently to prevent any appreciable condensation of solvent vapors thereon and the movement of the article from the heating section into said pool being sufficiently rapid as to prevent substantial cooling of the article before it is dipped into said pool of coating material and the temperature and time of movement of the article through the upper portion being such as to substantially solidify the coating material thereon.

9. A method of coating an article with solvent-containing liquid coating material, comprising passing the article through a heated zone to preheat the article to a temperature above the condensation point of vaporiz'ed solvent from the material, dipping it while still so heated into a pool of the heated, solvent-containing liquid coating material, the pre-heating of the article being suicient to prevent any appreciable condensation of solvent vapors thereon as it approaches said pool of coating material, then passing it through a cooled zone for promoting removal of solvent from the liquid coating material on the article, and then repassing the coated article through said heated zone.

10. A method of coating articles, comprising isolating a section containing a pool of heated solvent-containing liquid coating material, preheating each article of a succession of articles, passing it into said section through a cooled zone, dipping it into the pool, the heating being sulicient to prevent any appreciable condensation of solvent vapors thereon before it is dipped into said pool of coating material, the movement of the article into said pool being sufliciently rapid as to prevent substantial cooling of the article, and then passing it through a cooled zone for promoting removal of solvent from the liquid coating material on the article.

1l. A method of coating articles, comprising isolating a section containing a pool of heated, solvent-containing liquid coating material, preheating each article of a succession of articles to substantially the temperature of the coating material in the pool, passing it into said section through a cooled zone, dipping it into the pool, the heating being suicient to prevent any appreciable condensation of solvent vapors thereon before it is dipped into said pool of coating material, the movement of the article into said pool being suliiciently rapid as to prevent substantial cooling of the article, and then passing it through a cooled zone for promoting removal of solvent from the liquid coating material on the article, this latter movement being beneath cooling means and at a substantial angle to the vertical for preventing drops of excess coating material from one article falling on following articles.

12. A method of coating an article, comprising isolating a pool of solvent-containing liquid coating material, heating the liquid in the pool to create a solvent-laden atmosphere above the pool, preheating the article to a ternperature above the condensation point of the solvent in said atmosphere, passing the article through said atmosphere While the temperature of said article remains above said condensation point to prevent condensation of vaporized solvent thereon, and then immediately dipping the arl ticle into said pool.

ReferencesCted in the tile of this patent UNITED STATES PATENTS 1,339,400 Lander May 11, 1920 1,585,880 Schnell May 25, 1926 2,269,592 McGraw Jan. 13, 1942 2,362,397 Pearce Nov. 7, 1944 2,447,840 Boss Aug. 24, 1948 2,488,156 Bamberger Nov. 15, 1949 2,515,489 Borushko July 18, 1950 2,550,232 Donnell Apr. 24, 1951 2,620,769 Ornitz Dec. 9, 1952 

9. A METHOD OF COATING AN ARTICLE WITH SOLVENT-CONTAINING LIQUID COATING MATERIAL, COMPRISING PASSING THE ARTICLE THROUGH A HEATED ZONE TO PRHEAT THE ARTICLE TO A TEMPERATURE ABOVE THE CONDENSATION POINT OF VAPORIZED SOLVENT FROM THE MATERIAL, DIPPING IT WHILE STILL SO HEATED INTO A POOL OF THE HEATED, SOLVENT-CONTAINING LIQUID COATING MATERIAL, THE PRE-HEATING OF THE ARTICLE BEING SUFFICIENT TO PREVENT ANY APPRECIABLE CONDENSATION OF SOLVENT VAPORS THEREON AS IT APPROACHES SAID POOL OF COATING MATERIAL, THEN PASSING IT THROUGH A COOLED ZONE FOR PROMOTING REMOVAL OF SOLVENT FROM THE LIQUID COATING MATERIAL ON THE ARTICLE, AND THEN REPASSING THE COATED ARTICLE THROUGH SAID HEATED ZONE. 